It is well known that one of the major limitations in cellular and PCS wireless telephone networks is the so-called co-channel interference. In the case of TDMA networks, such as GSM or NADC (otherwise known as “IS-136”), the co-channel interference is mainly caused by the fact that the spectrum allocated to the system is reused multiple times (“frequency reuse”). The problem may be more severe, or less, depending on the reuse factor, but in all cases a signal, received by a handset, will contain not only the desired forward channel from the current cell, but also signals originating in more distant cells. If the interference from a distant cell causes a degradation of the ability of the handset to receive correctly the desired signal, it becomes important to identify the source of co-channel interference and measure the relative strength of interference relative to the desired signal.
The known art in the field of drive-test instrumentation systems attempt to solve the above problem by carrying out the decoding of the so-called “color code” contained in the signals of every cell in the system. An example of such system is the one from Agilent Technologies for the drive-test measurements of the GSM cellular networks. When used during a drive test on board of a moving vehicle, the test system determines the signal strength and color code (called in the case of GSM “base station identification code”, or BSIC) for the desired signal, as well as the ratio of this signal's strength to the total strength of all co-channel interfering signals at regular frequent intervals on the vehicle path. In addition, at each measurement interval, it tries to decode the BSIC of one (dominating) co-channel interfering signal. The idea is that if all determinations were correct, they would provide the identification of interfering signals for the whole area covered by the drive test.
There are several reasons why the described method of co-channel interference measurement and identification has a severely limited utility.
One problem is that, since by definition the interfering co-channel signals are below the power level of the desired signal, the decoding of the color code of such signals is a difficult task. In absence of any processing gain associated with the decoding of the color code, the only approach available for this is some variation of the joint decoding of the desired signal and interfering signal or signals. There is a body of work describing such joint methods, but all share a common feature: to be useful, they have to be extremely complex in terms of the accuracy of the used channel models and exponential increase in the number of required operations versus the accuracy of the models and number of signals decoded. This common feature necessitates a truncation of the channel models in practical systems as well as limits in practice the number of jointly demodulated signals by two. Then there is no wonder that such systems have difficulty decoding color codes (BSIC in the case of GSM) in most cases, especially in presence of multipath and on board a moving vehicle. They work more reliably in a lab, although in a narrow range of relative power. The result is that the coverage of the interference information during a drive test is intermittent. The processing time of test instruments when they perform co-channel measurements and identification is fairly long and restricts the completeness of the coverage even further.
Another problem is that since color codes, including the BSIC in the GSM case, are not unique to base stations, but are repeated periodically, even when decoded, they provide limited identification capability in terms of establishing firmly which distant cell is the source of interference.
Furthermore, as was already mentioned, practical systems provide only information on one interfering co-channel signal. In practice, it is desirable to identify multiple interfering signals and measure their relative powers.
Thus, what is needed is a method and apparatus suitable for the measurement and unequivocal identification of several interfering co-channel signals with high reliability and completeness, and which would not require frequent use of processing-intensive and inefficient joint detection algorithms.